Digital laminography is used to inspect predominantly flat objects—where the extension in two directions is very great compared with the extension in the third direction—with a view to the 3-dimensional capture of a desired or undesired internal structure. It is used when simple radiography (direct or oblique radiography) does not allow an adequate representation of the 3D structure and other representation techniques, e.g. computed tomography, cannot be used. Laminography represents a further development of multi-angle radiography, comparable results are generated with limited angle CT, a special type of computed tomography, wherein the nature of the image reconstruction, i.e. the allocation of the individual projections to the volume imaging, differs.
Laminography systems are known from the state of the art. An overview of different designs of laminography systems can be found in DE 38 54 865 T2. A design is represented in U.S. Pat. No. 4,211,927 in connection with the properties of the imaging components, wherein the positioning is limited to a linear movement. Image acquisition in the case of digital laminography uses a synchronized, contrary movement or positioning of radiation source and detector relative to the object. The method is called linear laminography in the case of a linear movement and rotational laminography in the case of a movement on circular paths.
The movement occurs, in the case of both linear and rotational laminography, precisely about a virtual reference point, most often positioned in the object to be inspected, the movement plane is usually parallel to the surface of the object. The radiographic angle is defined, in conjunction with the distances between focus and object and between object and detector, by the length of displacement in the case of linear laminography and by the radius of the circular movement in the case of rotational laminography. This decisively influences the depth resolution in the radiation direction, thus the image quality and the detail recognition in the reconstructed laminograms.
In the case of linear laminography, because of the linear movement and the fluoroscopic images to be obtained thereby, only those structures that are not, or not nearly oriented in the plane defined by the direction of movement and the radiation direction—thus most often errors—can be captured in three dimensions. The more the orientation of the structure deviates from this named plane, the more spatial information the image contains. For the complete examination of the predominantly flat objects, a rotational laminography is therefore preferably carried out.
In the case of the known systems for carrying out linear or rotational laminography with moving imaging components, there are clear limitations both in respect of the method (path determined by the system geometry) and because of the rigidity and precision requirements to be met by the mechanical support structure, all the more so as the size and weight of the objects and/or the imaging components increase.
A decisive criterion for the image quality and (spatial) resolution is that the geometry of the imaging system is determined exactly, both in itself and in relation to the object for examination. Radiation source and detector are therefore usually coupled in a mechanical unit, the so-called C bend. If the object for examination is then rotated in the image plane in order to set the different irradiation directions in a plane lying parallel to the object, a rotational laminography data set is produced. If the C bend moves about a point of rotation lying in the object in a plane perpendicular to the object, a data set for limited angle CT is generated.
The dimensions and the bearing load of the C bend are to be matched to the object to be inspected and the components of the imaging system. As size and load increase, this problem can only be solved with substantial outlay. In addition, because of the optimized rigidity of the C bend, an alteration of the distance between focus and detector in order to change the magnification is to be realized only with high outlay. Many systems are therefore limited beforehand to a circular arc-shaped movement with a reference axis of rotation lying in the object. If a system which realizes a rotational laminography with a reference axis perpendicular to the object without rotation of the object is to be realized, the C bend must be cardanically suspended.
None of the known laminography systems allows travel along freely defined paths in the space (e.g. as any combination of linear and rotary movements), in particular not accompanied by an alteration—even simultaneous—of the distance between focus and detector (e.g. to travel along a path laid on a universal ball joint).
A further disadvantage especially of the X-ray rotational laminography systems available hitherto on the market which have no generators co-rotating with the radiation source is that an ad hoc critical or life-limiting twisting of the high-voltage cable results. This is the case in particular when large circle radii are to be travelled. In this case, it is as a rule necessary that the cone of rays of the radiation source is repositioned through a tilting, in order that the detector is still illuminated. The forces to be absorbed during such a twisting also limit the achievable mechanical precision.
A further limitation in rotational laminography systems produced according to known designs for high-power radiation sources, for example linear accelerators, is that on the one hand they cannot be operated in all directions and on the other hand, because of their size and their weight, they can be co-moved in a (in particular cardanically suspended) C arm on complicated, three-dimensional paths only with substantial outlay.